Luminaires and lighting systems for general illumination typically contain one or more light-emitting diodes (LEDs) or other illumination sources that each emit a single color or correlated color temperature (CCT), but lighting systems can include multiple such sources whose outputs combine to provide an overall CCT, color, or illumination spectrum. Controlling the relative outputs of the different sources allows the user to obtain either the individual CCTs or theoretically any mixed combination thereof. This process is herein termed “color mixing” or “color tuning.” For convenience, the terms “CCT,” “color,” and “spectrum” are herein used interchangeably to refer to the spectrum of light emitted by an illumination source. Applications for color mixing are numerous, and include color adjustment to influence mood, perception, learning, and productivity, as well as to convey information.
Conventionally, luminaires featuring LEDs or other illumination sources are commonly dimmed (i.e., brightness-modulated) using any of a variety of techniques, for example increasing or decreasing the power (for example current or voltage) to the LEDs or modulating the power to the LEDs, for example pulse-with modulation (PWM) of the current or voltage.
The overall brightness and overall color of a luminaire that includes multiple LED colors may be modulated by separately modulating the brightness of the LED colors. For example, the output of a luminaire having red, green, and blue LEDs may be made bluer by reducing the power supplying the red and green LEDs relative to the power supplying the blue LED, and may be made dimmer, for any given color mix, by proportionately reducing the power supplying all three LED colors.
However, conventional techniques for adjusting the brightness and color output of a luminaire featuring LED arrays have several limitations and drawbacks. FIG. 1 schematically depicts portions of an illustrative lighting system 100 according to one conventional technique for controlling the brightness and color balance of an LED luminaire. System 100 features a luminaire or lighting system 102 having two different color LEDs 106 and 108. When powered, a first LED (or group of LEDs) 106 radiates light at a first CCT or color, herein termed Color A, and a second LED (or group of LEDs) 108 emits light at a second characteristic CCT or color, herein termed Color B. A first power supply 110 supplies power to Color A LED 106 through wires 114 and 116, and a second power supply 112 supplies power to Color B LED 108 through wires 118 and 120. To adjust the color of the overall output of the luminaire 102, the outputs of the power supplies are raised or lowered relative to each other: for example, if the output of the first power supply 110 is significantly higher than that of the second power supply 112, Color A will dominate the emission spectrum of luminaire 102. Decreasing the outputs of both power supplies 110, 112 while maintaining the outputs' relative magnitudes will cause the luminaire 102 to produce dimmer light of an approximately fixed color. Thus, color mixing and dimming of the luminaire 102 requires adjusting the outputs of the two power supplies 110, 112.
Thus, for a system of M different color LEDs, M separate power supplies need to be provided and separately controlled. Another drawback of conventional techniques is that 2M dedicated wires must typically be run from each power supply to each luminaire or array of luminaires having M distinctive LED colors, in order to provide a separately controllable current loop for each color.
Accordingly, there is a need for techniques by which color mixing and dimming of a luminaire featuring arrays of lighting sources having various CCTs may be achieved using fewer power supplies and fewer wires.